Electrosurgical system and method having enhanced temperature measurement

ABSTRACT

Electrosurgical systems and methods are described herein in which the temperature of a fluid within a body or joint space is determined and/or monitored despite the energy generated during treatment by an ablation probe. One or more temperature sensors are positioned along the probe proximally of the electrode assembly and measure the temperature of an electrically conductive fluid without being overly influenced by the surgical effect occurring proximate the electrode assembly. A controller automatically suspends energy delivery for one or more periods of time while the temperature is monitored.

FIELD OF THE INVENTION

The present invention relates to systems and methods for measuringtemperatures during electrosurgical procedures within a body space of apatient body, such as within a joint. More particularly, the presentinvention relates to methods and apparatus for measuring temperatures ofan electrically conductive fluid within a body space during ablation,such as within a joint space, without being significantly influenced bythe surgical effect initiated at the active electrode.

BACKGROUND OF THE INVENTION

The field of electrosurgery includes a number of loosely relatedsurgical techniques which have in common the application of electricalenergy to modify the structure or integrity of patient tissue.Electrosurgical procedures usually operate through the application ofvery high frequency currents to cut or ablate tissue structures, wherethe operation can be monopolar or bipolar. Monopolar techniques rely ona separate electrode for the return of RF current, that is placed awayfrom the surgical site on the body of the patient, and where thesurgical device defines only a single electrode pole that provides thesurgical effect. Bipolar devices comprise both electrodes for theapplication of current between their surfaces.

Electrosurgical procedures and techniques are particularly advantageoussince they generally reduce patient bleeding and trauma associated withcutting operations. Additionally, electrosurgical ablation procedures,where tissue surfaces and volume may be reshaped, cannot be duplicatedthrough other treatment modalities.

Present electrosurgical techniques used for tissue ablation suffer froman inability to control the depth of necrosis in the tissue beingtreated. Most electrosurgical devices rely on creation of an electricarc between the treating electrode and the tissue being cut or ablatedto cause the desired localized heating. Such arcs, however, often createvery high temperatures causing a depth of necrosis greater than 500 μm,frequently greater than 800 μm, and sometimes as great as 1700 μm. Theinability to control such depth of necrosis is a significantdisadvantage in using electrosurgical techniques for tissue ablation,particularly in arthroscopic procedures for ablating and/or reshapingfibrocartilage, articular cartilage, meniscal tissue, and the like.

Generally, radiofrequency (RF) energy is extensively used duringarthroscopic procedures because it provides efficient tissue resectionand coagulation and relatively easy access to the target tissues througha portal or cannula. However, a typical phenomenon associated with theuse of RF during these procedures is that the currents used to inducethe surgical effect can result in heating of electrically conductivefluid used during the procedure to provide for the ablation and/or toirrigate the treatment site. If the temperature of this fluid wereallowed to increase above a threshold temperature value, the heatedfluid could result in undesired necrosis or damage to surroundingneuromuscular and/or soft tissue structures.

Previous attempts to mitigate these damaging effects have includedeither limiting the power output of the RF generator or to include asuction lumen on the distal tip of the electrosurgical device tocontinuously remove the affected fluid from the surgical site andthereby reduce the overall temperature. These solutions may be effectivebut are limited and they do not allow for direct feedback based upon theactual temperature of the fluid within the joint space. Furthermore,limiting the power output of the generator reduces the rate of thesurgical effect, which is often unacceptable from a clinicalperspective. The incorporation of a suction lumen to allow heated fluidto be removed also reduces the performance of the electrosurgicaldevice.

There have been numerous RF based systems introduced into the marketthat make use of a temperature sensor (e.g., a thermocouple) in order tomonitor the temperature of tissue at or near the electrode.

However, the temperature sensors are susceptible to electrical noise.Electrical noise may arise from a number of sources including, forexample, (1) high frequency noise present on the electrical circuit usedto measure the small voltages induced by the temperature sensor, namely,a thermocouple, or (2) resistive heating of the thermocouple junctionarising from the delivery of the ablative energy to the tissue.

Filtering the measured signal to reject the high frequency componentscan generally remove the high frequency noise described above. However,the error arising from the resistive heating of the thermocouplejunction is a physical phenomena that cannot be mitigated by filtering.An improved system and method to accurately monitor the temperature ofthe fluid is still desired.

SUMMARY OF THE INVENTION

During the electrosurgical ablation of tissue wherein an electrosurgicalprobe comprising a temperature sensor is positioned in electricallyconductive fluid in the region of the target tissue, a controller isoperative to receive a temperature signal from the temperature sensorpositioned in the electrically conductive fluid. The controller isfurther operable to automatically suspend or reduce delivery of the highfrequency energy to the active electrode terminal of the probe for oneor more suspension periods. The controller monitors the temperaturesignal during the suspension periods. The temperature of theelectrically conductive fluid at the target site is calculated orestimated by the controller based on the monitored temperature signal.During the suspension period, and while the energy is suspended orsubstantially lowered, the temperature signal stabilizes therebyenhancing temperature measurement.

The duration of the suspension period may be fixed, or varied and basedon feedback from the procedure. In one embodiment the suspension periodcontinues until the change in temperature is less than a thresholdvalue. In another embodiment the suspension period is fixed at a valueequal to or greater than 250 ms.

In another embodiment, the controller is operable to suspend delivery ofhigh frequency energy for a plurality of suspension periods. Theplurality of suspension periods are determined by the controlleraccording to a suspension frequency (F) equal to the number ofsuspension periods per second. The suspension frequency (F) may becalculated or set by the controller using one or more techniques oralgorithms. In one embodiment the controller is adapted to receive aninput corresponding to a surgical procedure and the suspension frequency(F) is determined based on said input. In another embodiment thesuspension frequency (F) is based on a power output. In anotherembodiment the suspension frequency (F) is based on the measuredtemperature and is increased when the measured temperature reaches athreshold temperature. Typically, the frequency (F) oftemperature-monitoring periods shall be between ⅓ and 2.

In another embodiment, the controller includes a means to detachably andelectrically couple with the electrosurgical probe. This may be in theform of an input jack, or receptacle. In another embodiment thecontroller includes a microprocessor for controlling the power supplyand an analog-to-digital converter for converting the temperature signalto a digital signal readable by the microprocessor.

In another embodiment the system includes the electrosurgical probe andthe probe comprises one or more fluid delivery and/or aspirationelements. The fluid delivery element directs fluid to the target site.The fluid delivery element is coupled to a pump which is operable tocontrol fluid inflow to the target site. The controller is operable tocontrol the pump and the fluid inflow in order to maintain thetemperature of the fluid below a predetermined level. The system mayalso include a fluid aspiration lumen wherein the fluid aspiration lumenis a component of the electrosurgical probe.

In another embodiment, a method for ablating tissue at a target sitecomprises positioning a distal end of an electrosurgical instrumentadjacent to the tissue to be treated. High frequency energy is appliedby the instrument. The method further includes sensing a temperature ofthe electrically conductive fluid in the vicinity of the tissue andautomatically adjusting or suspending the step of applying the highfrequency energy while continuing to sense the temperature. Adjustingthe energy delivery may be performed by, for example, substantiallylowering or suspending the delivery of energy.

In the step of suspending applying high frequency energy, the durationof the suspension period may be fixed, vary, or based on feedback fromthe procedure. In one embodiment the suspension period continues untilthe change in temperature is less than a threshold value. In anotherembodiment the suspension period is fixed at a value equal to or greaterthan 250 ms.

In another embodiment, the step of suspending comprises suspending theapplying step for plurality of suspension periods according to asuspension frequency (F). The suspension frequency (F) is at least 1period every 3 seconds and less than 2 periods per second.

In another embodiment, the suspension frequency (F) is determined basedon power output. In another embodiment, the suspension frequency (F) isdetermined based on receiving an input of a type of surgical procedureto be performed. In another embodiment, the suspension frequency (F) isdetermined based on sensing the temperature, and the suspensionfrequency (F) is increased once the temperature reaches a thresholdlimit.

The method may further include a step of circulating fluid to the targetsite at a flowrate adjusted by the controller. The measured temperatureis compared to a desired temperature range and the flowrate is adjustedbased on the measured temperature.

In another embodiment, the applying step forms a plasma in the vicinityof the active electrode terminal of the electrosurgical probe therebycausing ablation of the soft tissue.

In another embodiment, the method is performed wherein the target siteis a joint.

The description, objects and advantages of the present invention willbecome apparent from the detailed description to follow, together withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the electrosurgical system including anelectrosurgical probe and electrosurgical power supply.

FIG. 2 is side view of an electrosurgical probe according to the presentembodiments.

FIG. 3 is a cross-sectional view of the electrosurgical probe of FIG. 2.

FIG. 4A is a perspective view of an embodiment of the active electrodefor the probe of FIGS. 1 and 2.

FIG. 4B is a detailed view of the distal tip of the electrosurgicalprobe of FIGS. 1 and 2 incorporating the active screen electrode of FIG.4A.

FIG. 5 illustrates a detailed view illustrating ablation of tissue.

FIG. 6A is a partial cross-sectional side view of a temperature sensorpositioned along the shaft of an electrosurgical probe proximally of theelectrode assembly.

FIG. 6B is a detail cross-sectional side view of a temperature sensorinsulated via an adhesive.

FIG. 7 is a side view of another variation where multiple temperaturesensors may be positioned about the shaft of an electrosurgical probeproximally of the electrode assembly.

FIG. 8 is a side view of yet another variation in which a temperaturesensor may be integrated along the shaft of an electrosurgical probe.

FIG. 9 is a side view of yet another variation where a temperaturesensor may be positioned within a fluid lumen of an electrosurgicalprobe to sense the fluid temperature immediately removed from thevicinity of the active electrode.

FIG. 10 is a schematic representation of a microcontroller within thecontroller which is coupled to the temperature sensor.

FIG. 11 is an illustrative graph showing how the microcontroller may beprogrammed comparing treatment time versus temperature.

FIG. 12 is an illustrative graph showing how the microcontroller may beprogrammed to indicate an alarm at a first temperature threshold and tocease further power upon the temperature reaching a second temperaturethreshold.

FIG. 13 is a schematic representation of a microcontroller and a fluidpump which may be used to control the inflow or outflow of fluidsthrough an electrosurgical probe to control temperature.

FIG. 14A is an illustrative graph showing measured temperature rise anddecline as the flow rate of the fluid is varied.

FIG. 14B is an illustrative graph showing increases in flow rate basedupon the sensed temperature.

FIG. 15 is an illustrative graph showing energy output versus time.

FIG. 16 is an illustrative graph showing temperature versus time.

DETAILED DESCRIPTION OF THE INVENTION

Before the present invention is described in detail, it is to beunderstood that this invention is not limited to particular variationsset forth herein as various changes or modifications may be made to theinvention described and equivalents may be substituted without departingfrom the spirit and scope of the invention. As will be apparent to thoseof skill in the art upon reading this disclosure, each of the individualembodiments described and illustrated herein has discrete components andfeatures which may be readily separated from or combined with thefeatures of any of the other several embodiments without departing fromthe scope or spirit of the present invention. In addition, manymodifications may be made to adapt a particular situation, material,composition of matter, process, process act(s) or step(s) to theobjective(s), spirit or scope of the present invention. All suchmodifications are intended to be within the scope of the claims madeherein.

Methods recited herein may be carried out in any order of the recitedevents which is logically possible, as well as the recited order ofevents. Furthermore, where a range of values is provided, it isunderstood that every intervening value, between the upper and lowerlimit of that range and any other stated or intervening value in thatstated range is encompassed within the invention. Also, it iscontemplated that any optional feature of the inventive variationsdescribed may be set forth and claimed independently, or in combinationwith any one or more of the features described herein.

All existing subject matter mentioned herein (e.g., publications,patents, patent applications and hardware) is incorporated by referenceherein in its entirety except insofar as the subject matter may conflictwith that of the present invention (in which case what is present hereinshall prevail). The referenced items are provided solely for theirdisclosure prior to the filing date of the present application. Nothingherein is to be construed as an admission that the present invention isnot entitled to antedate such material by virtue of prior invention.

Reference to a singular item, includes the possibility that there areplural of the same items present. More specifically, as used herein andin the appended claims, the singular forms “a,” “an,” “said” and “the”include plural referents unless the context clearly dictates otherwise.It is further noted that the claims may be drafted to exclude anyoptional element. As such, this statement is intended to serve asantecedent basis for use of such exclusive terminology as “solely,”“only” and the like in connection with the recitation of claim elements,or use of a “negative” limitation. Last, it is to be appreciated thatunless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs.

The treatment device of the present invention may have a variety ofconfigurations. However, one variation of the device employs a treatmentdevice using Coblation® technology.

The assignee of the present invention developed Coblation® technology.Coblation® technology involves the application of a high frequencyvoltage difference between one or more active electrode(s) and one ormore return electrode(s) to develop high electric field intensities inthe vicinity of the target tissue. The high electric field intensitiesmay be generated by applying a high frequency voltage that is sufficientto vaporize an electrically conductive fluid over at least a portion ofthe active electrode(s) in the region between the tip of the activeelectrode(s) and the target tissue. The electrically conductive fluidmay be a liquid or gas, such as isotonic saline, blood, extracelluar orintracellular fluid, delivered to, or already present at, the targetsite, or a viscous fluid, such as a gel, applied to the target site.

When the conductive fluid is heated enough such that atoms vaporize offthe surface faster than they recondense, a gas is formed. When the gasis sufficiently heated such that the atoms collide with each othercausing a release of electrons in the process, an ionized gas or plasmais formed (the so-called “fourth state of matter”). Generally speaking,plasmas may be formed by heating a gas and ionizing the gas by drivingan electric current through it, or by shining radio waves into the gas.These methods of plasma formation give energy to free electrons in theplasma directly, and then electron-atom collisions liberate moreelectrons, and the process cascades until the desired degree ofionization is achieved. A more complete description of plasma can befound in Plasma Physics, by R. J. Goldston and P. H. Rutherford of thePlasma Physics Laboratory of Princeton University (1995), the completedisclosure of which is incorporated herein by reference.

As the density of the plasma or vapor layer becomes sufficiently low(i.e., less than approximately 1020 atoms/cm³ for aqueous solutions),the electron mean free path increases to enable subsequently injectedelectrons to cause impact ionization within the vapor layer. Once theionic particles in the plasma layer have sufficient energy, theyaccelerate towards the target tissue. Energy evolved by the energeticelectrons (e.g., 3.5 eV to 5 eV) can subsequently bombard a molecule andbreak its bonds, dissociating a molecule into free radicals, which thencombine into final gaseous or liquid species. Often, the electrons carrythe electrical current or absorb the radio waves and, therefore, arehotter than the ions. Thus, the electrons, which are carried away fromthe tissue towards the return electrode, carry most of the plasma's heatwith them, allowing the ions to break apart the tissue molecules in asubstantially non-thermal manner.

By means of this molecular dissociation (rather than thermal evaporationor carbonization), the target tissue structure is volumetrically removedthrough molecular disintegration of larger organic molecules intosmaller molecules and/or atoms, such as hydrogen, oxygen, oxides ofcarbon, hydrocarbons and nitrogen compounds. This moleculardisintegration completely removes the tissue structure, as opposed todehydrating the tissue material by the removal of liquid within thecells of the tissue and extracellular fluids, as is typically the casewith electrosurgical desiccation and vaporization. A more detaileddescription of this phenomena can be found in commonly assigned U.S.Pat. No. 5,697,882 the complete disclosure of which is incorporatedherein by reference.

In some applications of the Coblation® technology, high frequency (RF)electrical energy is applied in an electrically conducting mediaenvironment to shrink or remove (i.e., resect, cut, or ablate) a tissuestructure and to seal transected vessels within the region of the targettissue. Coblation® technology is also useful for sealing larger arterialvessels, e.g., on the order of about 1 mm in diameter. In suchapplications, a high frequency power supply is provided having anablation mode, wherein a first voltage is applied to an active electrodesufficient to effect molecular dissociation or disintegration of thetissue, and a coagulation mode, wherein a second, lower voltage isapplied to an active electrode (either the same or a differentelectrode) sufficient to heat, shrink, and/or achieve hemostasis ofsevered vessels within the tissue.

The amount of energy produced by the Coblation® device may be varied byadjusting a variety of factors, such as: the number of activeelectrodes; electrode size and spacing; electrode surface area;asperities and sharp edges on the electrode surfaces; electrodematerials; applied voltage and power; current limiting means, such asinductors; electrical conductivity of the fluid in contact with theelectrodes; density of the fluid; and other factors. Accordingly, thesefactors can be manipulated to control the energy level of the excitedelectrons. Since different tissue structures have different molecularbonds, the Coblation® device may be configured to produce energysufficient to break the molecular bonds of certain tissue butinsufficient to break the molecular bonds of other tissue. For example,fatty tissue (e.g., adipose) has double bonds that require an energylevel substantially higher than 4 eV to 5 eV (typically on the order ofabout 8 eV) to break. Accordingly, the Coblation® technology generallydoes not ablate or remove such fatty tissue; however, it may be used toeffectively ablate cells to release the inner fat content in a liquidform. Of course, factors may be changed such that these double bonds canalso be broken in a similar fashion as the single bonds (e.g.,increasing voltage or changing the electrode configuration to increasethe current density at the electrode tips). A more complete descriptionof this phenomena can be found in commonly assigned U.S. Pat. Nos.6,355,032; 6,149,120 and 6,296,136, the complete disclosures of whichare incorporated herein by reference.

The active electrode(s) of a Coblation® device may be supported withinor by an inorganic insulating support positioned near the distal end ofthe instrument shaft. The return electrode may be located on theinstrument shaft, on another instrument or on the external surface ofthe patient (i.e., a dispersive pad). The proximal end of theinstrument(s) will include the appropriate electrical connections forcoupling the return electrode(s) and the active electrode(s) to a highfrequency power supply, such as an electrosurgical generator.

In one example of a Coblation® device for use with the embodimentsdisclosed herein, the return electrode of the device is typically spacedproximally from the active electrode(s) a suitable distance to avoidelectrical shorting between the active and return electrodes in thepresence of electrically conductive fluid. In many cases, the distaledge of the exposed surface of the return electrode is spaced about 0.5mm to 25 mm from the proximal edge of the exposed surface of the activeelectrode(s), preferably about 1.0 mm to 5.0 mm. Of course, thisdistance may vary with different voltage ranges, conductive fluids, anddepending on the proximity of tissue structures to active and returnelectrodes. The return electrode will typically have an exposed lengthin the range of about 1 mm to 20 mm.

A Coblation® treatment device for use according to the presentembodiments may use a single active electrode or an array of activeelectrodes spaced around the distal surface of a catheter or probe. Inthe latter embodiment, the electrode array usually includes a pluralityof independently current-limited and/or power-controlled activeelectrodes to apply electrical energy selectively to the target tissuewhile limiting the unwanted application of electrical energy to thesurrounding tissue and environment resulting from power dissipation intosurrounding electrically conductive fluids, such as blood, normalsaline, and the like. The active electrodes may be independentlycurrent-limited by isolating the terminals from each other andconnecting each terminal to a separate power source that is isolatedfrom the other active electrodes. Alternatively, the active electrodesmay be connected to each other at either the proximal or distal ends ofthe catheter to form a single wire that couples to a power source.

In one configuration, each individual active electrode in the electrodearray is electrically insulated from all other active electrodes in thearray within the instrument and is connected to a power source which isisolated from each of the other active electrodes in the array or tocircuitry which limits or interrupts current flow to the activeelectrode when low resistivity material (e.g., blood, electricallyconductive saline irrigant or electrically conductive gel) causes alower impedance path between the return electrode and the individualactive electrode. The isolated power sources for each individual activeelectrode may be separate power supply circuits having internalimpedance characteristics which limit power to the associated activeelectrode when a low impedance return path is encountered. By way ofexample, the isolated power source may be a user selectable constantcurrent source. In this embodiment, lower impedance paths willautomatically result in lower resistive heating levels since the heatingis proportional to the square of the operating current times theimpedance. Alternatively, a single power source may be connected to eachof the active electrodes through independently actuatable switches, orby independent current limiting elements, such as inductors, capacitors,resistors and/or combinations thereof. The current limiting elements maybe provided in the instrument, connectors, cable, controller, or alongthe conductive path from the controller to the distal tip of theinstrument. Alternatively, the resistance and/or capacitance may occuron the surface of the active electrode(s) due to oxide layers which formselected active electrodes (e.g., titanium or a resistive coating on thesurface of metal, such as platinum).

The Coblation® device is not limited to electrically isolated activeelectrodes, or even to a plurality of active electrodes. For example,the array of active electrodes may be connected to a single lead thatextends through the catheter shaft to a power source of high frequencycurrent.

The voltage difference applied between the return electrode(s) and theactive electrode(s) will be at high or radio frequency, typicallybetween about 5 kHz and 20 MHz, usually being between about 30 kHz and2.5 MHz, preferably being between about 50 kHz and 500 kHz, often lessthan 350 kHz, and often between about 100 kHz and 200 kHz. In someapplications, applicant has found that a frequency of about 100 kHz isuseful because the tissue impedance is much greater at this frequency.In other applications, such as procedures in or around the heart or headand neck, higher frequencies may be desirable (e.g., 400-600 kHz) tominimize low frequency current flow into the heart or the nerves of thehead and neck.

The RMS (root mean square) voltage applied will usually be in the rangefrom about 5 volts to 1000 volts, preferably being in the range fromabout 10 volts to 500 volts, often between about 150 volts to 400 voltsdepending on the active electrode size, the operating frequency and theoperation mode of the particular procedure or desired effect on thetissue (i.e., contraction, coagulation, cutting or ablation.)

Typically, the peak-to-peak voltage for ablation or cutting with asquare wave form will be in the range of 10 volts to 2000 volts andpreferably in the range of 100 volts to 1800 volts and more preferablyin the range of about 300 volts to 1500 volts, often in the range ofabout 300 volts to 800 volts peak to peak (again, depending on theelectrode size, number of electrons, the operating frequency and theoperation mode). Lower peak-to-peak voltages will be used for tissuecoagulation, thermal heating of tissue, or collagen contraction and willtypically be in the range from 50 to 1500, preferably 100 to 1000 andmore preferably 120 to 400 volts peak-to-peak (again, these values arecomputed using a square wave form). Higher peak-to-peak voltages, e.g.,greater than about 800 volts peak-to-peak, may be desirable for ablationof harder material, such as bone, depending on other factors, such asthe electrode geometries and the composition of the conductive fluid.

As discussed above, the voltage is usually delivered in a series ofvoltage pulses or alternating current of time varying voltage amplitudewith a sufficiently high frequency (e.g., on the order of 5 kHz to 20MHz) such that the voltage is effectively applied continuously (ascompared with, e.g., lasers claiming small depths of necrosis, which aregenerally pulsed about 10 Hz to 20 Hz). In addition, the duty cycle(i.e., cumulative time in any one-second interval that energy isapplied) is on the order of about 50% for the present invention, ascompared with pulsed lasers which typically have a duty cycle of about0.0001%.

The preferred power source may deliver a high frequency currentselectable to generate average power levels ranging from severalmilliwatts to tens of watts per electrode, depending on the volume oftarget tissue being treated, and/or the maximum allowed temperatureselected for the instrument tip. The power source allows the user toselect the voltage level according to the specific requirements of aparticular neurosurgery procedure, cardiac surgery, arthroscopicsurgery, dermatological procedure, ophthalmic procedures, open surgeryor other endoscopic surgery procedure. For cardiac procedures andpotentially for neurosurgery, the power source may have an additionalfilter, for filtering leakage voltages at frequencies below 100 kHz,particularly frequencies around 60 kHz. Alternatively, a power sourcehaving a higher operating frequency, e.g., 300 kHz to 600 kHz may beused in certain procedures in which stray low frequency currents may beproblematic. A description of one suitable power source can be found incommonly assigned U.S. Pat. Nos. 6,142,992 and 6,235,020, the completedisclosure of both patents are incorporated herein by reference for allpurposes.

The power source may be current limited or otherwise controlled so thatundesired heating of the target tissue or surrounding (non-target)tissue does not occur. In a presently preferred embodiment of thepresent invention, current limiting inductors are placed in series witheach independent active electrode, where the inductance of the inductoris in the range of 10 μH to 50,000 μH, depending on the electricalproperties of the target tissue, the desired tissue heating rate and theoperating frequency. Alternatively, capacitor-inductor (LC) circuitstructures may be employed, as described previously in U.S. Pat. No.5,697,909, the complete disclosure of which is incorporated herein byreference. Additionally, current-limiting resistors may be selected.Preferably, these resistors will have a large positive temperaturecoefficient of resistance so that, as the current level begins to risefor any individual active electrode in contact with a low resistancemedium (e.g., saline irrigant or blood), the resistance of the currentlimiting resistor increases significantly, thereby minimizing the powerdelivery from said active electrode into the low resistance medium(e.g., saline irrigant or blood).

Moreover, other treatment modalities (e.g., laser, chemical, other RFdevices, etc.) may be used in the inventive method either in place ofthe Coblation® technology or in addition thereto.

Referring now to FIG. 1, an exemplary electrosurgical system forresection, ablation, coagulation and/or contraction of tissue will nowbe described in detail. As shown, certain embodiments of theelectrosurgical system generally include an electrosurgical probe 120connected to a power supply 110 for providing high frequency voltage toone or more electrode terminals on probe 120. Probe 120 includes aconnector housing 144 at its proximal end, which can be removablyconnected to a probe receptacle 132 of a probe cable 122. The proximalportion of cable 122 has a connector 134 to couple probe 120 to powersupply 110 at receptacle 136. Power supply 110 has an operatorcontrollable voltage level adjustment 138 to change the applied voltagelevel, which is observable at a voltage level display 140. Power supply110 also includes one or more foot pedals 124 and a cable 126 which isremovably coupled to a receptacle 130 with a cable connector 128. Thefoot pedal 124 may also include a second pedal (not shown) for remotelyadjusting the energy level applied to electrode terminals 142, and athird pedal (also not shown) for switching between an ablation mode anda coagulation mode.

Referring now to FIG. 2, an electrosurgical probe 10 representative ofthe currently described embodiments includes an elongate shaft 13 whichmay be flexible or rigid, a handle 22 coupled to the proximal end ofshaft 13 and an electrode support member 14 coupled to the distal end ofshaft 13. Probe 10 includes an active electrode terminal 12 disposed onthe distal tip of shaft 13. Active electrode 12 may be connected to anactive or passive control network within a power supply and controller110 (see FIG. 1) by means of one or more insulated electrical connectors(not shown). The active electrode 12 is electrically isolated from acommon or return electrode 17 which is disposed on the shaft proximallyof the active electrode 12, preferably being within 1 mm to 25 mm of thedistal tip. Proximally from the distal tip, the return electrode 17 isgenerally concentric with the shaft of the probe 10. The support member14 is positioned distal to the return electrode 17 and may be composedof an electrically insulating material such as epoxy, plastic, ceramic,glass or the like. Support member 14 extends from the distal end ofshaft 13 (usually about 1 to 20 mm) and provides support for activeelectrode 12.

Referring now to FIG. 3, probe 10 may further include a suction lumen 20for aspirating excess fluids, bubbles, tissue fragments, and/or productsof ablation from the target site. Suction lumen 20 extends throughsupport member 14 to a distal opening 21, and extends through shaft 13and handle 22 to an external connector 24 (see FIG. 2) for coupling to avacuum source. Typically, the vacuum source is a standard hospital pumpthat provides suction pressure to connector 24 and suction lumen 20.Handle 22 defines an inner cavity 18 that houses electrical connections26 and provides a suitable interface for electrical connection to powersupply/controller 110 via an electrical connecting cable 122 (see FIG.1).

In certain embodiments, active electrode 12 may comprise an activescreen electrode 40. Screen electrode 40 may have a variety of differentshapes, such as the shapes shown in FIGS. 4A and 4B. Electricalconnectors 48 (see FIG. 9) extend from connections 26 through shaft 13to screen electrode 40 to electrically couple the active screenelectrode 40 to the high frequency power supply 110 (see FIG. 1). Screenelectrode 40 may comprise a conductive material, such as tungsten,titanium, molybdenum, platinum, or the like. Screen electrode 40 mayhave a diameter in the range of about 0.5 to 8 mm, preferably about 1 to4 mm, and a thickness of about 0.05 to about 2.5 mm, preferably about0.1 to 1 mm. Screen electrode 40 may comprise a plurality of apertures42 configured to rest over the distal opening 21 of suction lumen 20.Apertures 42 are designed to allow for the passage of aspirated excessfluids, bubbles, and gases from the ablation site and are typicallylarge enough to allow ablated tissue fragments to pass through intosuction lumen 20. As shown, screen electrode 40 has a generallyirregular shape which increases the edge to surface-area ratio of thescreen electrode 40. A large edge to surface-area ratio increases theability of screen electrode 40 to initiate and maintain a plasma layerin conductive fluid because the edges generate higher current densities,which a large surface area electrode tends to dissipate power into theconductive media.

In the representative embodiment shown in FIGS. 4A and 4B, screenelectrode 40 includes a body 44 that rests over insulative supportmember 14 and the distal opening 21 to suction lumen 20. Screenelectrode 40 further comprises at least five tabs 46 that may rest on,be secured to, and/or be embedded in insulative support member 14. Incertain embodiments, electrical connectors 48 (see FIG. 9) extendthrough insulative support member 14 and are coupled (i.e., viaadhesive, braze, weld, or the like) to one or more of tabs 46 in orderto secure screen electrode 40 to the insulative support member 14 and toelectrically couple screen electrode 40 to power supply 110 (see FIG.1). Preferably, screen electrode 40 forms a substantially planar tissuetreatment surface for smooth resection, ablation, and sculpting of themeniscus, cartilage, and other soft tissues. In reshaping cartilage andmeniscus, the physician often desires to smooth the irregular, raggedsurface of the tissue, leaving behind a substantially smooth surface.For these applications, a substantially planar screen electrodetreatment surface is preferred.

Further details and examples of instruments which may be utilized hereinare described in detail in U.S. Pat. Nos. 6,254,600; 6,557,559 and7,241,293 which are incorporated herein by reference in their entirety.

FIG. 5 representatively illustrates in more detail the removal of atarget tissue by use of an embodiment of a representativeelectrosurgical probe 50 according to the present disclosure. As shown,the high frequency voltage is sufficient to convert the electricallyconductive fluid (not shown) between the target tissue 502 and activeelectrode terminal(s) 504 into an ionized vapor layer 512 or plasma. Asa result of the applied voltage difference between electrode terminal(s)504 and the target tissue 502 (i.e., the voltage gradient across theplasma layer 512), charged particles 515 in the plasma are accelerated.At sufficiently high voltage differences, these charged particles 515gain sufficient energy to cause dissociation of the molecular bondswithin tissue structures in contact with the plasma field. Thismolecular dissociation is accompanied by the volumetric removal (i.e.,ablative sublimation) of tissue and the production of low molecularweight gases 514, such as oxygen, nitrogen, carbon dioxide, hydrogen andmethane. The short range of the accelerated charged particles 515 withinthe tissue confines the molecular dissociation process to the surfacelayer to minimize damage and necrosis to the underlying tissue 520.

During the process, the gases 514 will be aspirated through a suctionopening and suction lumen to a vacuum source (not shown). In addition,excess electrically conductive fluid, and other fluids (e.g., blood)will be aspirated from the target site 500 to facilitate the surgeon'sview. During ablation of the tissue, the residual heat generated by thecurrent flux lines 510 (typically less than 150° C.) between electrodeterminals 504 and return electrode 511 will usually be sufficient tocoagulate any severed blood vessels at the site. If not, the surgeon mayswitch the power supply (not shown) into the coagulation mode bylowering the voltage to a level below the threshold for fluidvaporization, as discussed above. This simultaneous hemostasis resultsin less bleeding and facilitates the surgeon's ability to perform theprocedure.

Because of the energy generated and applied during treatment within thepatient body with the above-described probe 10 or other variationsthereof, difficulties arise in determining, monitoring, and/or limitingthe actual temperature of electrically conductive fluid irrigating thetreated body space, joint, or tissue region. Accordingly, probe 10 mayinclude mechanisms for measuring a temperature of the electricallyconductive fluid itself without being overly influenced by the surgicaleffect occurring at the active electrode 12. Turning to FIG. 6A, oneembodiment is illustrated in the side view of probe 10 and the detailside view showing a temperature sensor 70 positioned along the probeshaft proximally of the return electrode 17. Temperature sensor 70 maycomprise any number of sensors, e.g., thermocouple, thermistor,resistance temperature detector (RTD), etc. In particular, temperaturesensor 70 may comprise a T-type thermocouple as these sensors arewell-established for use in such probes.

To reduce or eliminate the temperature-monitoring influence from anactive electrode 12 during tissue treatment, sensor 70 is desirablydistanced from both the active electrode 12 and return electrode 17 andmay accordingly be positioned proximally along the shaft 13 of probe 10.In the example shown, the distance L₁ of sensor 70 removed from returnelectrode 17 is at least 5 mm but may also be less than or greater thanthis distance, as practicable. With sensor 70 positioned accordingly,the sensor 70 may measure the temperature of the infused electricallyconductive fluid/irrigant surrounding the probe 10 and sensor 70 as thetemperature of the fluid is indicative of the temperature of thesurrounding tissue or joint space within which probe 10 may bepositioned for treatment. The fluid temperature may thus be measuredwithout regard to any energy generated by the current traveling betweenactive electrode 12 and return electrode 17 of probe 10.

A method to improve temperature measurement and reduce noise includesadjusting (e.g., lowering or suspending) energy delivery for one or moreperiods of time while continuing to monitor the temperature signal. Thismay be performed, for example, using a controller 110 as shown anddescribed in FIG. 10.

The controller determines the period of time that the RF energy issuspended. The period of time is preferably sufficient for the noise todiminish, and for the temperature measurement to stabilize. In oneembodiment, the suspension period is set at a constant value equal to orgreater than 100 ms, and more preferably equal to or greater than 250ms.

The suspension period may also be determined as a function of time. Thecontroller allows or permits the suspension period to continue until thetemperature varies less than about 1 degree per 50 ms.

In one embodiment of the invention, the frequency (F) of the suspensionperiods (e.g., the number of periods per second) is determined by thecontroller. The frequency (F) may be a predetermined constant value.This value may be selected or programmed conservatively at, for example,2 periods per second or more.

In another embodiment, the controller operates to determine thefrequency of the suspension periods (F) based on the magnitude and/orvariability of the temperature, or the power output measured in realtime during a procedure. For example, in one embodiment of theinvention, at high temperatures or power outputs the controller operatesto increase the frequency of the suspension periods so that safety ofthe patient is not compromised. Similarly, when the temperature variesgreatly, the controller operates to increase the frequency of both thetemperature measurement and the suspension periods.

In another embodiment, the controller is programmed to receive an inputsignal from an operator corresponding to a type of procedure to beperformed. The controller determines the frequency (F) of the suspensionperiods based on the type of procedure. For example, a procedurerequiring coagulation would typically be associated with higher heatgeneration. The controller would thus increase the frequency ofsuspension periods or temperature-monitoring periods to increase theaccuracy and safety of the procedure. Should a temperature of theelectrically conductive fluid exceed a threshold temperature, the systemwould make adjustments to the power output or another component of thesystem in order to reduce the measured temperature.

In another embodiment, the controller includes a database containing aplurality of types of procedures, and a frequency and suspension periodcorresponding to each type of procedure. The controller receives aninput procedure signal and automatically determines the frequency andsuspension period according to a predetermined value from the database.

The above described frequency (F) typically is between one period every500 ms to one period every 3000 ms. However, the invention is notintended to be limited as such except as where specifically stated inthe appended claims.

FIGS. 15-16 illustrate a graphical representation of the application ofenergy and temperature versus time. FIG. 15 shows an RF output curve 300representative of the application of voltage or RF output in bursts orpulses. The energy is applied for a period of time (e.g., an activeperiod 304) and suspended for a period of time (suspension period 306).Curve 300 shows a plurality of suspension periods.

FIG. 16 shows an illustration of a temperature curve 310 including aplurality of noise areas 312 and noise-free areas 314. The noise areasand noise-free areas correspond to the active periods and suspensionperiods of FIG. 15 respectively.

As shown, the temperature stabilizes during the suspension periods 314.In contrast, during energy delivery periods 304 the temperaturefluctuates greatly illustrating the benefits of periodically suspendingapplication of energy while monitoring the temperature.

Temperature sensor 70 may be mounted directly upon the shaft asillustrated in FIG. 6A. However, certain embodiments of probe 10 mayhave a suction lumen (see FIG. 3) for aspirating fluid and ablativebyproducts from the treatment site, wherein the inflow and/or outflow offluid and gas through the underlying suction lumen may affect thetemperature sensed by sensor 70. Thus, a thermally insulative layer 74such as heat shrink tubing or other insulation (e.g., comprised ofthermoplastics, such as polyolefin, polyvinyl chloride (PVC),polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP),etc.) may be placed between the temperature sensor 70 and outer surfaceof shaft 13. Sensor 70 may be secured directly to the shaft 13 and/orunderlying layer 74 via another insulative layer 76 overlying sensor 70and conducting wire 72 coupled to sensor 70. This overlying insulativelayer prevents the temperature of the surrounding fluid from effectingthe measurement at sensor 70. The addition of the overlying layer 76,which may be comprised of any of the materials mentioned above, may alsoelectrically isolate temperature sensor 70 from its surrounding salineenvironment to prevent or inhibit electrical noise from being introducedinto the temperature measurement circuit. Overlying layer 76 may beadhesive lined to further isolate the sensor 70.

Additionally and/or alternatively, temperature sensor 70 may be isolatedand secured to the underlying layer 74 by an adhesive 78, e.g., epoxy orcyanoacrylate glue, which may be adhered directly upon sensor 70, asillustrated in the detail side view of FIG. 6B.

In another embodiment, a side view of FIG. 7 shows a variation wheremultiple temperature sensors 70, e.g., greater than one sensor, may bepositioned around the shaft 13 to obtain multiple readings of the fluidtemperature. Although the multiple temperature sensors 70 may beuniformly positioned relative to one another about a circumference ofshaft 13, they may be alternatively positioned at arbitrary locations aswell. Moreover, each of the multiple sensors 70 may be positioned atdiffering distances L₁ along shaft 13 from return electrode 17. Insensing the multiple fluid temperatures, each of the temperatures may bedisplayed to the user and/or alternatively they may be calculated topresent an average temperature value to the user and/or the maximum ofthe measured values may be displayed.

In yet another variation, a side view of FIG. 8 shows another variationwhere temperature sensor 70 may be integrated along the shaft 13 suchthat sensor 70 may be recessed along the shaft surface and conductingwire 72 may be passed through a lumen (not shown) defined through probe10. Sensor 70 may still be insulated from the shaft 13 and may also beinsulated as described above. In such an embodiment, a hole throughshaft 13 may be located at the location of sensor 70 to improve theaccuracy of the measurement of the fluid external to shaft 13.

Referring now to FIG. 9, in yet another variation a representative probe10 having a suction lumen 20 for aspirating electrically conductivefluid from the body or joint space, a temperature sensor 70 andconducting wire 72 may be alternatively positioned within the suctionlumen 20 itself, as illustrated in the detail cross-sectional view ofFIG. 9. In this example, a temperature of the electrically conductivefluid recently in the immediate vicinity of the active screen electrode40 and then aspirated into suction lumen 20 may be measured as onemethod for determining a temperature-effect induced in nearby tissuesdue to the electrosurgical procedure. Such temperature measurementscould be used to control the RF output in order to provide therapieswhere it may be desirable to elevate the temperature of the targettissue to a specific temperature range. This configuration may alsoyield temperature data that may be used to directly correlate thetemperature of the target tissue from the aspirated conductivefluid/irrigant and thereby allow the user to get direct feedback of theactual temperature of the tissue and/or limit the RF output depending onpreset limits or for a given procedure or tissue type.

Independently from or in addition to the temperature sensing mechanismsin or along the probe 10, the power supply/controller 110 may also beconfigured for determining and/or controlling a fluid temperature withinthe body or joint space under treatment. FIG. 10 shows a representativeschematic of controller 110 with cable 122 coupled thereto. The one ormore conducting wires from their respective temperature sensors may berouted through cable 122 and into electrical communication withanalog-to-digital (ADC) converter 90 which may convert the output of thetemperature sensor to a digital value for communication withmicrocontroller 92. The measured and converted temperature value may becompared by microcontroller 92 to a predetermined temperature limitpre-programmed or stored within microcontroller 92 such that if themeasured temperature value of the conductive fluid irrigating the bodyor joint space exceeds this predetermined limit, an alarm or indicatormay be generated and/or the RF output may be disabled or reduced.Additionally and/or alternatively, the microcontroller 92 may beprogrammed to set a particular temperature limit depending upon the typeof device that is coupled to controller 110.

Furthermore, microcontroller 92 may also be programmed to allow the userto select from specific tissue or procedure types, e.g., ablation ofcartilage or coagulation of soft tissues, etc. Each particular tissuetype and/or procedure may have a programmed temperature limit pre-set inadvance depending upon the sensitivity of the particular anatomy toinjury due to an elevation in temperature.

In additional variations, the microcontroller 92 may be programmed tomonitor the exposure of a body or joint space to a specific elevatedfluid temperature level rather than limiting the treatment temperatureupon the instantaneous measured temperature value. For example, as thefluid treatment temperature increases, tissue necrosis typically occursmore rapidly; thus, microcontroller 92 may be programmed to generate analarm or indication based upon a combination of time-temperatureexposure. An exemplary chart 200 is illustrated in FIG. 11 which showsfirst temperature plot 202 indicating treatment of a body or joint spaceexposed to a irrigating conductive fluid at a first elevated temperaturelevel. Because of the relatively elevated fluid treatment temperature,the treatment time may be limited to a first predetermined time 204 bymicrocontroller 92 which may shut off or reduce the power levelautomatically. This is compared to second temperature plot 206indicating treatment of a body or joint space exposed to a irrigatingconductive fluid at a second elevated temperature level which is lessthan first temperature plot 202. Because of the lower relativetemperature, tissue necrosis may occur at a relatively slower rateallowing the treatment time to be extended by microcontroller 92 to arelative longer time period to second predetermined time 208.

In yet another variation, microcontroller 92 may be programmed toincorporate a set of multiple progressive temperature limits, as shownin the exemplary chart of FIG. 12. A first temperature limit 212 may beprogrammed whereby if the measured temperature rise 210 of theirrigating conductive fluid in the body or joint space exceeded firstlimit 212, an alarm or indication may be automatically generated bymicrocontroller 92 to alert the user. A second temperature limit 214 mayalso be programmed whereby if the measured temperature 210 of theirrigating conductive fluid in the body or joint space exceeded thesecond limit 214, microcontroller 92 may be programmed to reduce ordeactivate the RF output of active electrode 12 to mitigate the risk ofinjury to the patient.

Additionally and/or alternatively, controller 110 may be furtherconfigured to interface directly with a fluid pump, e.g., an arthroscopysaline pump 220 which provides a controlled in-flow of electricallyconductive fluid (e.g., saline) to the body or joint space. Such a fluidpump 220 may be configured to provide control of both electricallyconductive fluid in-flow to the body or joint space as well as out-flowfrom the body or joint space, as shown in the schematic illustration ofFIG. 13. As illustrated, pump 220 may be electrically coupled to pumpcontroller 222 which in turn may be in communication withmicrocontroller 92. Pump 220 may be further fluidly coupled to fluidreservoir 224 which holds the electrically conductive fluid and/or anempty reservoir (not shown) for receiving evacuated electricallyconductive fluid from the body or joint space.

The measured temperature 230 of fluid within the body or joint space maybe monitored and utilized as a control parameter for the fluid pump 220whereby the fluid in-flow and/or out-flow may be regulated to maintain atemperature of the body or joint space within a specified range or belowa temperature limit where potential injury could occur. An example ofthis is illustrated in the chart of FIG. 14A, which shows the measuredtemperature 230 of fluid within the body or joint space increasingtowards a pre-programmed temperature limit 232. Once the measuredtemperature 230 has approached 234, 236 or exceeded this limit 232, thefluid pump 220 flow rate may be automatically increased bymicrocontroller 92 from a first pump flow rate 240 to a second increasedflow rate 242 until the measured temperature 230 decreases, at whichpoint the pump flow rate may be automatically decreased to the firstpump flow rate 240, as indicated in FIG. 14B. This temperaturemoderation may be continued by cycling the flow rates between an initiallevel and an increased level for the duration of the procedure if sodesired. Alternatively, the out-flow rate may be increased to remove anyheated fluid to lower the temperature of fluid within the body or jointspace.

Other modifications and variations can be made to the disclosedembodiments without departing from the subject invention. For example,other uses or applications are possible. Similarly, numerous othermethods of controlling or characterizing instruments or otherwisetreating tissue using electrosurgical probes will be apparent to theskilled artisan. Moreover, the instruments and methods described hereinmay be utilized in instruments for various regions of the body (e.g.,shoulder, knee, etc.) and for other tissue treatment procedures (e.g.,chondroplasty, menectomy, etc.). Thus, while the exemplary embodimentshave been described in detail, by way of example and for clarity ofunderstanding, a variety of changes, adaptations, and modifications willbe obvious to those of skill in the art. Therefore, the scope of thepresent invention is limited solely by the appended claims.

While preferred embodiments of this invention have been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the scope or teaching herein. The embodimentsdescribed herein are exemplary only and are not limiting. Because manyvarying and different embodiments may be made within the scope of thepresent teachings, including equivalent structures or materialshereafter thought of, and because many modifications may be made in theembodiments herein detailed in accordance with the descriptiverequirements of the law, it is to be understood that the details hereinare to be interpreted as illustrative and not in a limiting sense.

1. An electrosurgical system for treating tissue at a target site with an electrosurgical probe, the probe comprising a shaft having a distal end and a proximal end, an active electrode terminal disposed near the distal end, said system comprising: a high frequency power supply for delivery of high frequency energy to said active electrode terminal, the high frequency power supply coupled to the active electrode terminal and a return electrode; a controller for receiving a temperature signal from a temperature sensor positioned in an electrically conductive fluid located at the target site and wherein the electrically conductive fluid provides a current path between the active electrode terminal and the return electrode and wherein the controller is operable to automatically suspend delivery of the high frequency energy to said active electrode terminal for at least one suspension period, and said controller further being operable to monitor the temperature signal during the at least one suspension period and wherein said controller is adapted to determine a measured temperature of said electrically conductive fluid located at the target site based on said temperature signal being monitored.
 2. The system of claim 1 wherein said suspension period continues until the change in temperature signal versus time is less than a threshold value.
 3. The system of claim 2 wherein said threshold value is 1 degree per 50 ms.
 4. The system of claim 1 wherein said at least one suspension period is at least 250 ms.
 5. The system of claim 1 wherein said controller is operable to suspend delivery of high frequency energy for a plurality of suspension periods.
 6. The system of claim 5 wherein said plurality of suspension periods are determined by the controller according to a suspension frequency (F) equal to the number of suspension periods per second and wherein said suspension frequency (F) is between ⅓ and
 2. 7. The system of claim 1 wherein said controller comprises a means to detachably and electrically couple with the electrosurgical probe.
 8. The system of claim 6 wherein said controller is adapted to receive an input corresponding to a surgical procedure and said suspension frequency (F) is determined based on said input.
 9. The system of claim 6 wherein said controller is adapted to determine said suspension frequency (F) based on a power output.
 10. The system of claim 6 wherein said controller is adapted to determine said suspension frequency (F) based on said measured temperature and to increase the suspension frequency (F) when said measured temperature reaches a threshold temperature.
 11. The system of claim 10 wherein said threshold temperature is 40 degrees Celsius.
 12. The system of claim 1 further comprising: a microprocessor for controlling the power supply; and an analog-to-digital converter for converting the temperature signal to a digital signal readable by the microprocessor.
 13. The system of claim 1, wherein said system comprises said probe, and said probe further comprises an aspiration lumen for removing fluid from said target site.
 14. The system of claim 13, wherein said probe further comprises a fluid delivery element for delivering fluid to said target site and said fluid delivery element being coupled to a pump wherein the pump is operable to control fluid inflow to the target site, and wherein said controller being operable to control said pump and the fluid inflow in order to maintain the temperature of the fluid below a predetermined level.
 15. An electrosurgical method for monitoring temperature during an electrosurgical procedure to ablate soft tissue comprising: delivering high frequency energy to an active electrode terminal located at the distal end of an electrosurgical probe, said active electrode terminal being positioned in electrically conductive fluid during the procedure and wherein a current flow path from the active electrode, through the electrically conductive fluid, to a return electrode is created when the high frequency energy is delivered; sensing a temperature of the electrically conductive fluid; and automatically suspending the delivering step for at least one suspension period while sensing said temperature.
 16. The method of claim 15 wherein said suspension period is at least 250 ms.
 17. The method of claim 15 wherein said step of automatically suspending comprises suspending the delivering step for a plurality of suspension periods.
 18. The method of claim 17 wherein the suspending step is performed according to a suspension frequency wherein said suspension frequency (F) is at least 1 period every 3 seconds and less than 2 periods per second.
 19. The method of claim 18 wherein said suspension frequency (F) is determined based on power output.
 20. The method of claim 18 wherein said suspension frequency (F) is determined based on receiving an input of a type of surgical procedure to be performed.
 21. The method of claim 18 wherein said suspension frequency (F) is determined based on sensing the temperature, and said suspension frequency (F) is increased once the temperature reaches a threshold limit.
 22. The method of claim 15 further comprising: circulating fluid to the target site at a flowrate; comparing the temperature to a desired temperature range; and adjusting the flowrate based on the temperature.
 23. The method of claim 15 wherein said applying step forms a plasma in the vicinity of the active electrode terminal.
 24. The method of claim 15 wherein said target site is a joint.
 25. The method of claim 15 wherein said automatically suspending step is performed by reducing the voltage difference between the active electrode terminal and the return electrode to less than 100 volts.
 26. An electrosurgical system for treating tissue at a target site with an electrosurgical probe, the probe comprising a shaft having a distal end and a proximal end, an active electrode terminal disposed near the distal end, said system comprising: a high frequency power supply for delivery of high frequency energy to said active electrode terminal, the high frequency power supply coupled to the active electrode terminal and a return electrode; a controller for receiving a temperature signal from a temperature sensor positioned in an electrically conductive fluid located at the target site and wherein the electrically conductive fluid provides a current path between the active electrode terminal and the return electrode and wherein the controller is operable to automatically modify delivery of the high frequency energy to said active electrode terminal for at least one temperature-monitoring period, and said controller further being operable to monitor the temperature signal during the at least one temperature-monitoring period and wherein said controller is adapted to determine a measured temperature of said electrically conductive fluid located at the target site based on said temperature signal being monitored during said temperature-monitoring period.
 27. The system of claim 26, wherein the controller is operable to automatically modify delivery of the high frequency energy for a plurality of temperature-monitoring periods, and wherein said automatic modification comprises substantially reducing the delivery of the high frequency energy.
 28. The system of claim 27, wherein reducing the delivery of the high frequency output comprises reducing the high frequency output to less than 100 volts. 